The present invention relates to a method for modifying the surface of a substrate by laser irradiation in the presence of reactive species. The present invention further relates to an apparatus for carrying out the aforementioned method.
Etching techniques are known in the art that involve selective removal of materials by a reaction with chemically active gases created by a radio frequency (RF) power-induced glow discharge maintained in the etching chamber. In such a "plasma etching" process, chemically reactive gases such as CF.sub.4, are generally used. The surface of the substrate to be etched is covered with a mask leaving the selected areas of the substrate exposed. The substrate to be etched is then inserted into a chamber containing the reactive gas. To create the plasma, an RF voltage may be applied across the gas to cause the gas to dissociate and form various species comprising positive and negative ions, reactive atoms such as fluorine, and radicals. The plasma then reacts with the surface and forms volatile products, thereby leaving an etched surface in the areas exposed by the mask. It is also known that by changing the gas and other reaction conditions, a plasma can be used for deposition of a layer on a substrate rather than etching.
In a paper entitled "Plasma Annealing of Ion Implanted Semiconductors," Appl. Phys. Lett., 39:622-24 (1981), Ianno et al. disclose the use of a plasma-generated electron beam to anneal damaged semiconductors. The apparatus used by Ianno et al. comprises a four-inch inside diameter pyrex cross with a planar aluminum cathode, a stainless steel ring anode and a vertically arranged magnetic lens. One side port of the cross operates as the gas inlet and the opposing side port is used to introduce a sample holder. A glow discharge is maintained between the cathode and the ring anode, and Ianno et al report that the cathode, when constantly bombarded by photons, ions, metastable molecules and other excited species, emits electrons which are accelerated toward the negative glow of the discharge in the direction of the ring anode. A significant fraction of these electrons are focused by the magnetic lens to generate an electron beam with sufficient energy to penetrate an implanted substrate deep enough to ensure good crystal regrowth.
It is also known in the art to use a laser as the sole energy source to decompose, either pyrolytically or photolytically, precursor gases which contain halogen atoms, thereby producing reactive species which can etch the surface of a substrate. Gas pressures greater than 1 torr were generally required in order to achieve reasonable etch rates with this technique.
In a paper entitled "Laser Enhanced Chemical Etching of Solid Surfaces," IBM J. Res. Develop. 26:145-50 (1982), Chuang discloses the etching of, inter alia, Si and SiO.sub.2 films by halogen-containing gases wherein a laser is used to provide the energy to excite molecules into higher vibrational states, thereby enhancing the process of dissociative chemisorption and subsequent surface reactions to form volatile products. As an alternative mechanism, it is proposed that the excitation of gas molecules create reactive radicals which become involved in surface reactions. Laser enhanced chemical etching of silicon and silicon dioxide with pulsed CO.sub.2 and continuous argon lasers was investigated.
Lasers have also been used to deposit materials like silicon dioxide on substrate surfaces. For example, Boyer et al disclose a laser-induced photochemical deposition of silicon dioxide from gas phase donor molecules at a rate of up to 3,000 angstroms per minute. "Laser-Induced Chemical Vapor Deposition of Silicon Dioxide," Appl. Phys. Lett. 40: 716-19 (1982). According to Boyer et al., gas phase SiH.sub.4 and N.sub.2 O molecules were excited and dissociated by means of an argon fluoride laser operated at 193 nm. Deposition of nearly stoichiometric SiO.sub.2 was achieved at low temperatures by way of direct photolytic reaction of the N.sub.2 O and SiH.sub.4 using a coherent beam of photons from the argon fluoride excimer laser.
In the paper entitled "Low-Temperature Growth of Polycrystalline Silicon and Germanium Films by Ultraviolet Laser Photo-Dissociation of Silane and Germane," Appl. Phys. Lett. 40: 183-85 (1982), Andraetta et al disclose the growth of polycrystalline silicon and germanium films on amorphous silicon dioxide substrates. The disclosed process is said to occur through the photodissociation of silane/nitrogen or germane/helium mixtures using a pulse excimer laser. In particular, an argon fluoride laser was operated at 193 nm and a krypton fluoride laser operated at 248 nm. Typically, a repetition frequency of 20 hertz was used, with the laser beam being focused at a point from about 1-4 centimeters in front of the growth chamber using a 7 centimeter focal length quartz lens. The experiments were conducted under a total pressure of 450 torr and it was determined that for silicon and germanium the film growth rate strongly depended upon laser wavelength and intensity.
R. Solankyi discloses the deposition of the refractory metals chromium, molybdenum, and tungsten via laser-induced gas phase photolysis of the respective metallic hexacarbonyls. "Laser Photo-Deposition of Refractory Metals," Appl. Phys. Lett. 38: 572-74 (1981).
U.S. Pat. No. 4,281,030 teaches the implantation of a source material into a target by way of a laser-produced particle flux. The application of laser radiation to a source material produces a stream of ions which flows outward from the source material and deposits a film of the material onto a collector substrate surface which is disposed in the path of the ion flux. The implantation of a source material into the target is said to be enhanced if the target substrate is first softened by way of an incident laser radiation.
Both laser-based and plasma-based processing methods present significant disadvantages. As previously noted, etching techniques relying on laser irradiation alone require the use of halogen-containing gas at relatively high pressures. Also, laser chemical etching is slower than conventional plasma etching in many cases, particularly in terms of the total amont of material to be processed in a given period of time. Plasma etching or deposition, on the other hand, is much harder to control, since surface modification can proceed, in a nonselective manner, where ever the plasma comes in contact with the substrate surface. In addition, plasma-induced modification is often accompanied by radiation damage to the substrate.
U.S. Pat. No. 4,183,780 teaches the use of vacuum ultraviolet light to control a process involving plasma etching and deposition. In accordance with the disclosed process, vacuum ultraviolet radiation (600 to 1,000 angstroms, or below 200 nm) is directed toward and absorbed by the plasma species disposed adjacent a substrate, which may be a silicon wafer that carries a silicon dioxide layer to be selectively etched. Both the plasma etching and deposition processes can be controlled on a real time basis by turning the vacuum ultraviolet light source on or off. Preferred plasma species for reactive ion etching comprise O.sub.2, F.sub.2, and stable organic halides such as CF.sub.4 which require wavelengths in the vacuum ultraviolet range for photoionization and dissociation. Deposition of the plasma species similarly requires that the incident radiation be in the vacuum ultraviolet region of the electromagnetic spectrum. Also, the disclosed process involves contact between the plasma and the substrate, and thus suffers the drawbacks of conventional plasma processing.